In the second half of 2022, BIT scientific research teams have made research progress in perovskite solar cells, transmembrane transport mechanism of cyclic dinucleotides and folates, construction of MOF nanosheet membrane, construction of porous ionomers for fuel cells, construction of heterodimensional superlattice, etc., which have been published inNatureandScience. Now, these 5 high-level theoretical articles are summarized as follows for the readers.

BIT’s new research result of perovskite solar cells is published inScience

On November 18, Prof. Chen Qi’s team published an article entitled Initializing Film Homogeneity to Retard Phase Segregation for Stable Perovskite Solar Cells inScience, which reveals the influence of initial homogeneity of perovskite film materials on the stability of films and devices. On this basis, highly efficient and stable perovskite solar cells are prepared. Prof. Chen Qi of School of Materials Science & Engineering of BIT is the corresponding author, and Bai Yang, associate professor of School of Materials Science & Engineering of BIT, Huang Zijian, a doctoral candidate of Peking University, Zhang Xiao and Lu Jiuzhou, postgraduates of Grade 2018 of BIT, and Niu Xiuxiu, 2019 doctoral candidate are the co-first authors. The cooperative units of this research include Peking University, Institute of Automation of Chinese Academy of Sciences, Institute of High Energy Physics of Chinese Academy of Sciences, Institute of Physics of Chinese Academy of Sciences, Beihang University, Institute of Semiconductors of Chinese Academy of Sciences, and Beijing Yaoneng Technology Co., Ltd.

The new energy technology of photovoltaic power generation is of great significance for achieving the goal of carbon neutrality. In recent years, optoelectronic solar cell devices based on organic-inorganic hybrid perovskite have enjoyed rapid development. The highest photoelectric conversion efficiency reported at present is close to 26%, and device stability is the key to its industrialization.

Due to its rich component space, the mixed component perovskite has outstanding advantages in regulating the semiconductor performance of materials and improving the efficiency and stability of devices. However, because of the introduction of multi-component, multiphase competition will occur in the process of material growth, resulting in uneven initial component distribution of the film. The development of the inhomogeneity of mixed cation perovskite thin films is studied. It is found that the non-uniform sites of the film at the nanoscale will develop rapidly under external stimulation, leading to more serious component distribution differentiation (Fig. 1), and eventually forming a thermodynamic stable phase separation that runs through the entire perovskite film, causing material degradation and device deactivation.

Fig. 1 Initial distribution of components of (A-H) perovskite thin films and their evolution under external stimulation. Phase separation of perovskite thin films driven by (I-N) thermodynamics.

Although there have been relevant reports on the micro-nano scale phase separation thermodynamics and atomic scale element migration behavior, there is still a lack of a unified model to systematically study the individual behavior of ions and atoms and the macro behavior of films in a unified context. In order to solve this problem, a mathematical model is developed that can be used to simulate the migration and aggregation of ions in perovskite based on the Schelling model, which studies group behavior in economics and physics. In combination with the prediction of density functional theory and experimental observation data, the phase separation behavior of thin films has been semi-quantitatively analyzed. As shown in Fig. 2, the simulation results of the model show that the initial homogeneity of the perovskite film has a significant impact on the aging behavior of the film, and the improvement of film homogeneity will significantly slow down its aging rate.

Fig. 2 (A) Theoretical calculation of phase separation of perovskite films. Schelling model simulation of aging process of (B-F) perovskite thin films.

Guided by the prediction results of the model, the solution colloid environment is effectively regulated and the film homogeneity is improved by introducing the weakly coordinated additive selenol into the perovskite precursor solution. The experimental results show that the films with improved homogeneity show good stability under thermal and light aging conditions, and no significant phase separation occurs during the experimental period (Fig. 3). At the same time, after further device optimization, the prepared solar cell devices show good photoelectric performance, and obtain 23.7% certification efficiency in the 1cm² devices. Under different temperature conditions, the device also shows good working stability under continuous illumination of LED light source.

Fig. 3 (A) Theoretical calculation of phase separation of perovskite films. Schelling model simulation of aging process of (B-F) perovskite thin films.

In addition, the conclusions about the initial state of homogeneity can be further extended from mixed cation system to mixed anion system. Therefore, mixed anion perovskite thin films with different initial homogeneity are prepared and their aging processes are studied, which expands the universality of the conclusion that “improving the homogeneity of the component distribution of perovskite thin films is conducive to improving the stability of thin films and devices” (Fig. 4).

Fig. 4 Aging behavior of (A-H) mixed anionic perovskite films.

BIT reveals transmembrane transport mechanism of cyclic dinucleotides and folates inNature

On October 20, Gao Ang’s team of the School of Life Science of BIT, and Zhang Liguo’s team and Gao Pu’s team from the Institute of Biophysics of the Chinese Academy of Sciences published an article entitled Recognition of Cyclic Dinucleotides and Folates by Human SLC19A1 inNature, describing the molecular mechanism of SLC19A1, a solute carrier family protein, to recognize cyclic dinucleotides, folates and anti-folates, which contributes to the mechanism analysis of SLC19A1 related diseases and the development and optimization of potential drugs. Zhu Yalan, a postdoctoral fellow of BIT, is the co-first author, Prof. Gao Ang is the co-corresponding author, and BIT is the co-corresponding unit.

SLC19A1 is a key transport protein of cyclic dinucleotides, folates and anti-folates drugs in human body. Cyclic dinucleotides are important second messenger molecules that widely exist in nature, which are first catalysed by the natural immune receptor cGAS after sensing the abnormal DNA signal in the cytoplasm, and then combines and activates the junction protein STING, thus triggering a broad spectrum of natural immune response in cells. Folates are members of the vitamin B folates and important nutrients in the basic metabolism of the body. Anti-folates drugs such as pemetrexed (PMX) and methotrexate (MTX) can inhibit folates metabolism in tumor cells, playing a huge role in cancer treatment. In view of the important functions of SLC19A1 in the transmembrane transport of cyclic dinucleotides, folates and anti-folates, the study of its substrate recognition mechanism will contribute to the mechanism analysis of SLC19A1 related diseases and the development and optimization of potential drugs.

Schematic diagram of SLC19A1 transport cyclic dinucleotides and folates

This study analyzes the substrate free electron microscopic structure of inward open SLC19A1 and the high-resolution electron microscopic structure of SLC19A1 in the inward open state with mammalian endogenous cyclic dinucleotides (2'3'- cGAMP), bacterial representative cyclic dinucleotides (3'3'-CDA), and clinical cyclic dinucleotide drugs (2'3'- CDAS) through the method of freezing electron microscopy and antibody assistance. Then, SLC19A1 is found to be a classical MFS folding transporter composed of 12 transmembrane helices (TM). Three kinds of cyclic dinucleotides are bound to the bottom of SLC19A1 polar cavity in the form of delicate dimer units, which is a new substrate recognition mode of SLC and MFS family proteins.

In addition, another group of mouse derived monoclonal antibodies is used to analyze the high-resolution freeze electron microscopic structure of SLC19A1 complex with the main reduced folic acid 5-MTHF and the new anti-folates drug PMX. Unlike cyclic dinucleotides, these two folates molecules bind in the middle and upper parts of SLC19A1 polar cavity in the form of monomers.

This research is the first to report that human SLC19A1 recognized the complex structure of multiple cyclic dinucleotides, folates and anti-folates, and reveal the molecular mechanism of SLC19A1 recognizing different substrates, which will provide guidance for the mechanism research and drug development of related diseases.

BIT’s research result of MOF Nanosheet membrane is published inScience

On October 21, 2022, Prof. Zhao Zhiping’s team published an article entitled High Flexible and Superhydrophobic MOF Nanosheet Membrane for Ultrafast Alcohol-Water Separation inScience, proposing a new idea of embedding crystal seeds in polymer substrate and then accurately constructing MOF nanosheet membrane by surface crystal induced growth method, which realizes the hierarchical construction of highly flexible super hydrophobic MOF membrane on the surface of polymer substrate, analyzes the crystal structure of nanosheet and its internal mass transfer channel, reveals the synergistic mechanism of polymer and nanosheet in the separation process, and breaks through the bottleneck of flexible MOF membrane preparation, providing theoretical basis and technical support for large-scale preparation and application. The first authors are Xu Lihao and Li Shenhui, doctoral students of the School of Chemistry and Chemical Engineering of BIT, the corresponding authors are Zhao Zhiping and Feng Yingnan, and BIT is the only unit to completed this research.

The separation process is the most concentrated link of energy consumption, investment and cost in the chemical industry, accounting for 40-70% of the investment and cost, and more than 10% of the world’s energy consumption, which while is also an indispensable link in the fields of energy, environment, food and biomedicine. Pervaporation membrane separation technology can save energy by 30-60%, with remarkable characteristics of high efficiency and energy saving, which is not only a key technology to support sustainable development, but also plays a pivotal role in achieving the goal of “carbon emission peak and carbon neutrality” in China. Breaking through the “trade off” game effect between permeability and selectivity of separation membranes, the development of high-performance separation membranes is the unremitting pursuit of scientists in membrane science and technology.

In recent years, substrate supported heteroepitaxial metal organic framework (MOF) membranes have shown great potential in separation applications. Most of the existing methods prepare MOF membranes on rigid inorganic substrates. In order to break through the technical bottleneck of difficult membrane amplification and poor flexibility in membrane module processing and manufacturing, Prof. Zhao Zhiping’s team has prepared a highly flexible MOF-NS membrane by solving the scientific problems that restrict the technical bottleneck.

In order to solve the interface bonding problem between the MOF layer and the polymer substrate, the research team blends ZIF-8 crystal seeds into the polymer casting solution, and skillfully prepares a polyvinylidene fluoride membrane (SEEDS/PVDF) embedded with “bud” crystal seeds in the polymer substrate by using the non-solvent induced phase separation (NIPS) method. The “bud” crystal seeds embedded in the polymer substrate not only become the “anchors” for the connection between MOF nanosheets and polymers, but also lay the foundation for the growth of nanosheets with unique petal-like structure. Based on this substrate, a complete cellular MOF nanosheet membrane (MOF-NS/PVDF) is prepared by inducing the localized growth of MOF. The crystal structure of MOF nanosheet and its internal mass transfer channels are analyzed by X-ray diffraction (XRD) and Monte Carlo molecular simulation, whose topological structure takes [Zn₂(MeIm)⁴]n with a thickness of 0.525 nm as a grid plane and contains 0.435 nm sub-nanometer level interlayer channels, revealing that the lattice distortion of ZIF-8 crystal seed occurred in the process of NIPS membrane formation.

Fig. 1 Structure and preparation method of MOF-NS/PVDF membrane: (A) surface morphology of the film (MOF-NS/PVDF membrane is prepared after 1h, 3h and 6h growth from SEEDS/PVDF substrate respectively); (B) schematic diagram of preparation of SEEDS/PVDF membrane and MOF-NS/PVDF membrane; (C) XRD spectra of PVDF membrane, SEEDS/PVDF membrane, MOF-NS/PVDF membrane and simulated [Zn₂(MeIm)⁴]n membrane.

Fig. 2 Molecular transport channels between layers of MOF-NS: (A) interlayer channels and pore size of Zn₂(MeIm)⁴; (B) MOF-NSHR-TEM image; (C) layer structure and interlayer channel of MOF-NS.

Under the electron microscope, by adjusting the electron beam bombardment density in the observation area, the reversible flexible deformation of the MOF nanosheet (that is, the twisting, overturning and swinging of the nanosheet) is captured for the first time. The thickness of the nanosheet is about 13 nm. The MOF nanosheet shows a good lattice structure different from ZIF-8 under the transmission electron microscope. The lamellar structure and internal continuous channels of the honeycomb MOF nanosheet make it show ultra-high permeability during the pervaporation process.

Fig. 3 High flexibility structure of MOF-NS/PVDF membrane: (A) SEM images of flexible reversible dynamic deformation process of MOF-NS/PVDF membrane (including overturning, twisting and swinging); (B) schematic diagram of flexible reversible dynamic deformation of MOF-NS/PVDF membrane; (C) bending test of MOF-NS/PVDF membrane; (D) SEM images of MOF-NS/PVDF membrane surface and section after bending.

MOF-NS/PVDF is modified by polydimethylsiloxane (PDMS) solution drop coating to form a PDMS coating with honeycomb structure, which not only repairs the molecular scale defects between MOF nanosheets, but also realizes the transformation of membrane surface characteristics from super hydrophilic to super hydrophobic (water contact angle 158.3°), and constructs a dual functional membrane (PDMS/MOF-NS/PVDF) with both super hydrophobic surface characteristics and MOF-NS fast molecular diffusion channels within the membrane.

Fig. 4 Preparation process, structure and surface characteristics of PDMS modified MOF-NS/PVDF membrane: (A) schematic diagram of drop coating process; (B) the change of the surface morphology of the membrane before and after drop coating.

The pervaporation separation test and molecular simulation of PDMS/MOF-NS/PVDF composite membrane reveal the synergistic mechanism of PDMS and MOF nanosheets in the ethanol-water separation process: the PDMS layer of organic compounds block the dissolution and permeation of water molecules and make ethanol molecules preferentially dissolve and penetrate; dimethylimidazole with lamellar structure in MOF nanosheet selectively adsorbs the alcohol molecules of PDMS, forming a secondary selection to improve the separation factor. At the same time, its internal continuous pore structure becomes a fast channel for molecular transfer, reducing the molecular transfer resistance. In addition, the membrane surface of the honeycomb structure increases the effective contact area with the feed liquid, promoting the increase of the permeation flux. In the process of separation, the sub-nanometer channels show the role of molecular sieve retention for larger butanol molecules. The PDMS-MOF nanosheet composite layer constructed on the polymer substrate not only enhances the molecular mass transfer in the membrane, but also effectively promotes the fluid turbulence near the membrane surface, reduces the concentration difference and temperature difference polarization phenomenon in the pervaporation process, thereby significantly improving the separation performance of the composite membrane. The permeation flux and separation factor are 13.6 times and 1.2 times of those of the PDMS/PVDF membrane prepared by traditional methods, respectively.

Fig. 5 Effect of membrane PV performance and membrane surface morphology on feed liquid flow behavior: (A) PV performance of membrane for separating 5 wt% ethanol aqueous solution at 40℃; (B) comparison of PV separation performance; (C) long-term stability of membrane; (D-E) flow behavior on the membrane surface.

Note: This research has been supported by the key programs of the National Natural Science Foundation of China, the national key research and development program, and the BIT young teacher academic startup program.

Breaking through inherent thinking, BIT’s research result of constructing porous ionomers for fuel cell for the first time is published inScience

On October 14, 2022, Prof. Wang Bo’s team published an article entitled Covalent Organic Framework Based Polar Ionomers for High Performance Fuel Cells inScience. In the field of hydrogen energy, a porous covalent organic framework (COF) ionomer suitable for the catalytic layer of fuel cells is proposed and constructed for the first time to solve the problems of ion conduction, gas and water transport and electrocatalysis in the gas solid liquid three-phase interface of the core membrane electrode assembly (MEA) of fuel cells. The concept of porous ionomers breaks through the bondage of traditional chain ionomers, can significantly improve the mass transfer efficiency of the catalyst layer, and greatly improve the power density of the fuel cell, making the mass activity of Pt/C catalyst and the peak power density of the fuel cell increased by 1.6 times. The first author of this article is Zhang Qingnuan, a postdoctoral fellow from the School of Chemistry and Chemical Engineering of BIT, the corresponding authors are Wang Bo and Feng Xiao, and BIT is the first author unit and the only corresponding unit.

Under the target of carbon neutrality, the development of hydrogen energy technology has become an inevitable trend. Proton exchange membrane fuel cell (PEMFC) is one of the most promising hydrogen energy utilization methods as the port of hydrogen energy scale utilization. The key to realize the development and large-scale application of PEMFC technology is to develop high-performance, low-cost MEA materials. The catalytic layer is composed of Pt/C catalyst and ionomer, and as the core of MEA, it is the place where the electrochemical reaction takes place in the fuel cell. In order to ensure the efficiency of the electrochemical reaction, the catalytic layer needs to simultaneously provide a channel for protons and reaction gases needed for the reaction to reach the catalyst, and also be able to transport the water molecules generated by the reaction. At present, the ionomer used in the catalytic layer is chain perfluorinated sulfonic acid resin (PFSA, Nafion), which can realize rapid proton conduction. At the same time, however, Nafion will cause excessive encapsulation of Pt catalyst, resulting in large gas resistance and low utilization of catalytic active sites, which will lead to the failure to give full play to the catalyst performance.

To solve the above problems, the strategy of constructing porous frame ionomer is proposed, and the gas-solid liquid three-phase interface on the catalyst surface is optimized, so that the development of high-performance proton exchange membrane fuel cell system with low platinum content is realized. Under the guidance of framework chemistry, the organic units are linked by covalent bonds to accurately customize and synthesize porous framework 2D polymers, which have good chemical stability, thermal stability and anti-swelling ability. The obtained 2D polymer planar structure is composed of a hexagonal framework extending indefinitely and periodically in the 2D direction. In the hexagonal framework, a sulfonic acid based cantilever is used to provide high proton transmission capacity, and the remaining space can provide sufficient channels for oxygen and water to facilitate transmission. Under the condition that commercial Pt/C is used as cathode catalyst and the catalyst content is only 0.07 mg Pt cm⁻², the mass activity of Pt/C catalyst and the peak power density of fuel cell are increased by 1.6 times by using porous COF ionomer, and the peak power density of H2-Air is 1.08 W cm⁻².

Fig. 1 Pt in fuel cell/ C@COF-Nafion diagram of catalytic layer and its action mechanism

In this article, the mechanism of porous COF ionomers in the catalytic layer of fuel cells is detailedly discussed, and the effects of mesoporous COF nanosheets on gas diffusion, proton transport and water management in the catalytic layer are analyzed. Compared with traditional chain ionomers, porous COF ionomers have the following advantages:

1) It is conducive to gas mass transfer. With the addition of porous COF ionomer, the mass transfer resistance of O₂in H₂-air battery at the limiting current density decreases by 40%. The oxygen permeability test shows that the oxygen permeability of the mixed substrate membrane of COF and Nafion is significantly improved than that of the pure Nafion membrane, and the gas passing ability can still be maintained under humidity.

2) It is with high proton conductivity and optimized water management. The proton conductivity of porous COF ionomer is slightly higher than that of Nafion. Its pore structure and water absorption and drainage ability are not only conducive to inhibiting the occurrence of water flooding under high power density, but also can help fuel cells show good performance under low humidity.

3) It could mitigate the poisoning of catalyst. The steric hindrance generated by porous COF nanosheets significantly reduces the over encapsulation of the ionomer on the Pt/C catalyst, alleviates the direct contact between sulfonic acid groups and Pt, helps to expose more Pt active sites, increases the electrochemical active surface area, and improves the mass activity of the catalyst.

Fig. 2 Comparison of cathode Pt load and peak power density of MEA based on commercial Pt/C catalyst with literature values

Through the design concept of porous ionomer, that is, introducing rigid development framework nanosheets with rich mesopores without sacrificing proton conductivity, the performance of fuel cells can be greatly improved. With ultra-low platinum content, its power density has reached a new record, which means that it is expected to reduce the cost of generating 1kW electricity by about one-third. In addition, the goal set by the Department of Energy (DOE) in 2025 is to reduce the total content of PGMs in membrane electrodes to 0.1 g kW⁻¹. After optimizing the structure of the catalyst layer using porous COF ionomers, commercial catalysts can approach this goal.

The good thermal stability, acid base stability and structure designability of COF also make it have broad application prospects in high-temperature fuel cells or alkaline fuel cells. The design strategy of porous COF ionomer is a milestone for optimizing ORR three-phase microenvironment of fuel cell catalyst layer.

Note: This research is also supported by key research and development programs of the Ministry of Science and Technology, general programs of the National Natural Science Foundation of China, excellent youth program, Beijing Science and Technology Program, and China Postdoctoral Science Foundation, etc.

BIT has broken through the traditional material structure cognition——the research result of building a “different dimension super structure” is published inNature

On August 31, 2022, Prof. Zhou Jiadong and Prof. Yao Yugui of the School of Physics of BIT, Prof. Wu Xiaosong of Peking University, Japanses Prof. Kazu Suenaga and Prof. Liu Zheng of Nanyang Technological University published an article entitled Heterodimensional Superlattice with In-plane Anomalous Hall Effect inNature, which is the first time to propose and build a new material with different dimensions. Based on the observation of in-plane anomalous Hall effect at room temperature, the successful construction of this structure breaks through the cognition of traditional materials and structures, and creates a new direction for the study of new materials and novel physical properties.

Superlattice structure is generally composed of different parent materials arranged in a certain period to form a new material different from the parent material. The material with this new structure is expected to show novel physical properties that the parent material does not have, such as ferromagnetism, ferroelectricity and superconductivity, which has potential application value in the fields of electronics, optoelectronics and new information devices. The traditional superlattice structure is mainly formed by materials with the same dimension according to a certain arrangement and combination, such as 3D-3D, 2D-2D and 1D-1D structures. At the same time, these traditional superlattice structures are mainly made by molecular beam epitaxy.

The discovery of 2D materials provides a new possibility for the construction of new superlattice structures. Structural construction refers to the construction of intercalated or different dimensional heterostructures from different 2D materials and materials with other structures by artificial construction or chemical vapor deposition. However, since superlattice structure was proposed and prepared in 1970, it is particularly difficult to realize the intrinsic superlattice structure composed of materials with different dimensions. In order to solve this problem, Prof. Zhou Jiadong and his fellows first propose and realize a new heterodimensional superlattice structure, which is composed of 2D VS2 and 1D VS arranged alternately to form a new 2D-1D intrinsic heterodimensional superlattice form. Based on the unique structure of the material, the super structure exhibits a large in-plane anomalous Hall effect at room temperature, which opens a new direction for building new substances and discovering new physical properties.

Fig. 1 Preparation and optical characterization results of VS2-VS heterostructure

Different from the preparation of traditional superlattice structures, the realization of this new type of heterostructure is a milestone:

1) Breakthrough in preparation methods: molecular beam epitaxy (MBE) is often used to prepare traditional superstructure; the preparation of the superstructure with 2D material as the parent material is mostly artificial construction, which makes it difficult to control the cleanliness of the material interface and observe the intrinsic properties of the substance. The one-step VLS growth mechanism of the superstructure provides a direction for building new substances.

2) Breakthrough in material cognition: 2D materials and 1D lines periodically overlap to form a stable heterostructure, whose thickness can be adjusted from tens of nanometers to several nanometers. It breaks through the traditional superstructure materials that can only form simple intercalation of materials with the same dimension or different materials, and promotes the development of the material field.

Fig. 2 Atomic structure of heterodimensional VS2-VS supercrystal

3) Breakthrough in physical properties: due to the existence of 1D VS and its mutual coupling with 2D VS2, 2D-1D heterostructure shows a large in-plane anomalous Hall effect at room temperature that is completely different from VS2 and V5S8, etc., which promotes the coupling between substances with different dimensions, and provides the possibility to realize novel physical properties, such as ferromagnetic semiconductor properties at room temperature, quantum anomalous Hall effect at high temperature, etc.

Fig. 3 In-plane anomalous Hall effect of VS2-VS heterostructure

Note: This research is supported by the key R&D program of the Ministry of Science and Technology, the general program of the National Natural Science Foundation of China, the National Youth Talent Plan, the key program of the National Natural Science Foundation of China, the innovation program of BIT, and the provincial and ministerial quantum laboratory.